The principal objective of this proposal is to determine the cellular and humoral factors that are required in the immune regulation of Heligmosomoides polygyrus (Nematodirus dubius) and Nippostrongylus brasiliensis in mice. This objective will be addressed by reconstitution of congenitally thymus-deficient (nude) mice with subpopulations of lymphoid cells and/or antiserum or its components. Depletion of class specific antibody synthesis by heterologous anti-isotype immunosuppression will be used to determine the relevant antibody classes in protective immunity to these intestinal nematodes.